1. Field of the Invention
The present invention relates to a polarization multiplexing and transmitting apparatus that generates polarization multiplexed light.
2. Description of the Related Art
In recent years, the realization of high capacity optical communication systems has been demanded to cope with the increase in information transmitted through a network. As a technique to accomplish such, multiplexing (MUX) that utilizes polarization, a physical quantity of light, is examined. Polarization multiplexing is performed by using a coupler to couple two light components having polarization states orthogonal to each other.
FIG. 14 is a conceptual view of light subjected to polarization multiplexing. As shown in FIG. 14, axes H and V represent a horizontal direction and a vertical direction orthogonal to each other. Polarization multiplexed light 1400 includes a horizontal signal component 1410 and a vertical signal component 1420, each having a varying intensity. Here, time-aligned polarization multiplexing where timings of the signal component 1410 having a varying intensity and the signal component 1420 having a varying intensity are in-phase is performed. A “signal component having a varying intensity” is also called an “intensity-varying signal component”.
FIG. 14 and the following explanation are based on the assumption that polarization multiplexed light is obtained by multiplexing components in a linearly polarization state orthogonal to each other. For example, a term “polarization direction” is used with respect to a polarization plane of linearly polarized light, and not only linearly polarized light but also elliptically polarized light or circularly polarized light may be actually used provided polarization states are orthogonal, and “polarization direction” should be read as “polarization state” in this case.
FIG. 15 is a conceptual view of light subjected to polarization multiplexing. As shown in FIG. 15, D. Van Den Borne, et. al., in “1.6-B/S/Hz Spectrally Efficient Transmission Over 1700 Km Of SSMF Using 40×85.6-Gb/S Pol. MUX-RZ-DQPSK”, JLT, Vol. 25, No. 1, 2007, describes transmission that is resistant to non-linear noise of an optical fiber and based on time-interleaved polarization multiplexing where signal components 1410 and 1420, each having a varying intensity and included in the polarization multiplexed light 1400, are staggered by an amount corresponding to ½ of a pulse repetition cycle in terms of time.
FIG. 16 is a block diagram of a conventional polarization multiplexing and transmitting apparatus. As shown in FIG. 16, a conventional polarization multiplexing and transmitting apparatus 1600 disclosed in the specification of a U.S. patent, U.S. Pat. No. 6,580,535, uses polarization adjusters 1621 and 1622 to orthogonalize polarization directions of respective signals having varying intensities output from optical senders 1611 and 1612 and also uses an optical adder 1630 to add these signals, thereby performing polarization multiplexing (see FIG. 14).
FIG. 17 is a block diagram of a conventional polarization multiplexing and transmitting apparatus. As shown in FIG. 17, a conventional polarization multiplexing and transmitting apparatus 1700 disclosed in the specification of a U.S. patent, U.S. Pat. No. 5,111,322, uses a splitter 1710 to split an RZ (Return-to-Zero) optical pulse stream output from a mode-locked laser 1710 (M.L.L.) into respective streams and modulators 1731 and 1732 (MOD.) to modulate the respective streams, and couples the modulated respective streams in a multiplexer 1740 (POL. SPLITTER), thereby effecting polarization multiplexing.
Here, a delay adjustment circuit 1750 is provided between the modulator 1731 and the multiplexer 1740, and the delay adjustment circuit 1750 is used to delay one of the respective streams for one pulse and thereby staggers the streams with respect to each other, thus executing time-interleaved polarization multiplexing (see FIG. 15). Here, the delay adjustment circuit 1750 is formed of plural mirrors that divert the passing respective streams while reflecting them.
However, as the conventional technology depicted in FIG. 16 provides for two optical senders (optical senders 1611 and 1612), two laser diodes (LD) to generate continuous wave light are also required. Therefore, this technology has a problem in that apparatus size and manufacturing cost increase. When continuous wave light generated by the LDs are converted into signals having varying intensities by an optical divider, the power of each signal having a varying intensity is attenuated to half of that of the continuous wave light. Therefore, this technology has a problem in that the power consumed to obtain a necessary power of a polarization multiplexed light doubles.
The conventional technology depicted in FIG. 17 has a problem in that apparatus size and manufacturing cost increase since the delay adjustment circuit 1750, which includes mechanically movable components, is provided. When modulation of polarization multiplexed light is performed in a bit rate variable mode to cope with various signal formats (e.g., synchronous digital hierarchy (SDH), optical transport network (OTN), or Ether), a time-lag of the respective streams must be adjusted according to a change in bit rate. When using the delay adjustment circuit 1750 to adjust the time-lag of the respective streams, this technology has a problem in that the configuration and control of the delay adjustment circuit 1750 are complicated.
To eliminate problems associated with conventional techniques, it is an object of the present invention to provide a polarization multiplexing and transmitting apparatus that can perform time-interleaved polarization must while reducing apparatus size.